Vacuum degassed, interstitial-free, low carbon steel and method for producing same

ABSTRACT

A low carbon, vacuum degassed steel containing columbium, titanium and/or zirconium, having high ductility, no yield point elongation in the hot rolled and cold rolled and annealed conditions. The steel contains from 0.002 to 0.020 percent carbon, up to 0.60 percent manganese, from greater than 0.025 to 0.12 percent columbium, titanium when present from 0.015 to 0.12 percent, zirconium when present from 0.028 to 0.18 percent, up to 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to 0.035 percent sulfur, up to 0.045 percent total aluminum, and balance substantially iron. The steel has extra-deep drawing quality.

United States Patent [191 Elias et al.

[ Oct. 16, 1973 VACUUM DEGASSED,

INTERSTITIAL-FREE, LOW CARBON STEEL AND METHOD FOR PRODUCING SAME [75 Inventors: James A. Elias, Middletown; Rollin E. Hook, Dayton, both of Ohio [73] Assignee: Armco Steel Corporation,

Middletown, Ohio [22] Filed: May 19, 1972 [21] Appl. No.: 255,108

[52] U.S. Cl. 75/123 H, 75/123 T, 75/123 M, 148/2, 148/12, 148/12.3, 148/3l.5, 148/36 [51] Int. Cl C22c 39/54, C2ld 7/02, C2ld 7/14 [58] Field of Search 75/123 .1, 123 H, 75/123 M; 148/36, 2,12, 12.3, 31.5

[5 6] References Cited UNITED STATES PATENTS 3,544,393 12/1970 Zanetti. 148/12 3,598,658 8/1971 Matsukura et a1 148/2 3,666,570 5/1972 Korchynsky et a1. 148/12 3,671,336 6/1972 Korchynsky et al. 148/12.3 3,673,007 6/1972 Miyano et al 148/12 Primary Examiner-W. W. Stallard Attorney.lohn W. Melville et a1.

[5 7] ABSTRACT A low carbon, vacuum degassed steel containing co- I lumbium, titanium and/or zirconium, having high duetility, no yield point elongation in the hot rolled and cold rolled and annealed conditions. The steel contains from 0.002 to 0.020 percent carbon, up to 0.60 percent manganese, from greater than 0.025 to 0.12 percent columbium, titanium when present from 0.015 to 0.12 percent, zirconium when present from 0.028 to 0.18 percent, up to 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to 0.035 percent sulfur, up to 0.045 percent total aluminum, and balance substantially iron. The steel has extra-deep drawing quality.

13 Claims, N0 Drawings VACUUM DEGASSED, INTERSTIT IAL-FREE, LOW CARBON STEEL AND METHOD FOR PRODUCING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to non-aging, low carbon, vacuum degassed steel containing columbium, titanium and/or zirconium, which has high ductility, a high tensile to yield strength ratio, and absence of yield point elongation in the hot rolled and cold rolled and annealed conditions. The steel has great utility for hot rolled, cold rolled box annealed, cold rolled continuously annealed, and cold rolled continuously annealed and hot dip metallic coated products, or nonmetallic coated products.

2. Description of the Prior Art Reference is made to application Ser. No. 107,077 filed Jan. 18, 1971, in the names of the present applicants, which discloses and claims a columbium treated, non-aging, vacuum degassed low carbon steel. The

steel of that application exhibits substantially no yield point elongation in the annealed condition, has excellent surface characteristics, substantial freedom from inclusions and freedom from critical grain growth. The steel consists essentially of from about 0.002 to about 0.015 percent carbon, from about 0.02 percent to about 0.30 percent columbium, from about 0.05 percent to about 0.60 percent manganese, sulfur up to about 0.035 percent, oxygen up to about 0.010 percent, nitrogen up to about 0.012 percent, aluminum up to about 0.080 percent, phosphorus and silicon in residual amounts, and remainder substantially iron.

Earlier workers in the art have disclosed the addition of columbium, tantalum, vanadium, boron, or titanium as carbide and/or nitride forming elements in order to obtain non-aging rimmed steel having good deep'drawing properties. Reference may be made to U.S. Pat. No. 2,999,749, issued Sept. 12, 1961, to E. R. Saunders et al., U.S. Pat. No. 3,102,831 issued Sept. 3, 1963, to N. F. Tisdale, and U.S. Pat. No. 3,183,078, issued May 11, 1965, to T. Ohtake et al., for summaries of earlier work in this field.

U.S. Pat. No. 3,522,110, issued July 28, 1970, to M. Shimizu et al., discloses a method of producing cold rolled steel alleged to be non-aging and to have excellent deep drawing properties. The steel contains from greater than 0.001 percent to less than 0.020 percent carbon, less than 0.45 percent manganese, less than 0.015 percent oxygen, less than 0.007 percent nitrogen, from greater than 0.02'percent to less than 0.5 percent titanium (except titanium present as titanium oxides), and the balance iron. The steel may contain sulfur in amounts of less than 0.05 percent and small amounts of aluminum. Titanium must be present in amounts greater than four times the carbon content. The process involves hot rolling the material at a temperature higher than 780 C (1,436 F), cold rolling with a reduction of more than 30 percent, and annealing at a temperature of from 650 to 1,000 C (1,202 to 1,832 F). Continuous annealing is stated to produce better properties in the product.

U.S. Pat. No. 3,607,456, issued Sept. 21, 1971, to J. L. Forand, Jr., discloses a steel alleged to have excellent deep drawing properties in the cold rolled and annealed condition and an ASTM grain size of6.0 to 9.0. The steel consists essentially of 0.020 percent maximum carbon, 0.60 percent maximum manganese, 0.010 percent maximum nitrogen, 0.015 percent maximum oxygen, 0.15 percent to 0.30 percent titanium, and balance essentially iron. A maximum of 0.03 percent sulfur may be present, and aluminum may be present in small amounts. The weight ratio of titanium to the sum of the carbon and nitrogen contents must be at least 7:1. The product is produced by hot rolling, finishing at a temperature above 1,550 F (843 C), cooling and coiling within a temperature range of 900 to 1,200 F (482 to 649 C), cold reducing by 50 to percent, and batch annealing within the temperature range of 1,550 F (843C) and the alpha-gamma transformation temperature.

British Pat. No. 1,192,794, in the name of Nippon Kokan l(.l(., published May 20, 1970, discloses a process of producing a low carbon steel alleged to be substantially non-aging and to have good deep drawing properties, which comprises reducing the carbon content of a rimmed molten steel to less than 0.02 percent by vacuum degassing, adding a carbide former, forming cold rolled sheets, and annealing the sheets at 700 to 950 C (1,292 to 1,742 F). The carbide former is titanium, vanadium, columbium, tantalum, zirconium, uranium, hafnium, or thorium, and must be added in sufficient amount to reduce the solute carbon content at the annealing temperature to less than 0.002 percent. In the case of titanium, the content must be more than four times the carbon content.

The above-mentioned patents make it clear that titanium has long been considered a highly effective element in eliminating aging and yield point elongation in low carbon steels. However, titanium-treated steels produced in accordance with the prior art processes have inherently rather low tensile strength in the cold rolled and annealed condition. This will be apparent from the data given hereinafter, wherein the average tensile strength of titanium-treated steels typical of the prior art is about 303 MN/m in the cold rolled and annealed condition.

The Forand patent contemplates the addition of an excess-of titanium, at. least part of which will go into solid solution. This is apparent from the requirement for a minimum of 0.15 percent titanium in Forand.

U.S. Pat. No. 3,102,831 issued Sept. 3, 1963 to N. F. Tisdale, discloses a method for production of killed, semi-killed, or rimmed steels containing from about 0.005 percent to about 0.050 percent columbium wherein ingots, slabs or bars are heated to a temperature above 2,300 F (1,260" C), hot rolled with a finishing temperature between 1,550and 1,750 F (843 and 955 C, cooled rapidly to less than 1,200 F (649 C), and then permitted to air cool at a normal rate. The steels contain from 0.02 to 0.50 percent carbon, from 0.005 to 0.5 percent silicon, from 0.15 to 1.6 percent manganese, 0.005 to 0.050 percent columbium, phosphorus and sulfur in residual amounts, and remainder iron.

U.S. Pat. No 2,999,749 issued Sept. 12, 1961 to E. R. Saunders et al., discloses a method for producing non-aging rimmed steel which includes adding to a molten steel an addition agent containing at least 25 percent manganese and at least one of columbium, tantalum, vanadium and boron in an amount sufficient to combine with the nitrogen present. Small amounts of a deoxidant such as zirconium, titanium, beryllium, magbe: greater than 0.025 wt. percent if nesium, aluminum, calcium, silicon and/or barium, may be incorporated in the addition agent.

While the above-mentioned copending application Ser. No. 107,007 discloses a material and process capable of a broad spectrum of properties in either the hot rolled or cold rolled conditions, the columbium addition results in precipitation hardening effects causing decreased ductility unless the carbon content is reduced to a low level and the hot rolled band is finished and coiled at a high temperature. Moreover, the product is relatively expensive by reason of the preferred columbium to carbon ratio of :1.

SUMMARY It is a principal object of the present invention to provide a non-aging low carbon steel having substantially no yield point elongation which in the hot rolled condition exhibits high ductility, good formability and low yield strength substantially independent of coiling temperature and of total carbon content, which in the cold rolled condition has excellent tensile elongation values and high tensile to yield strength ratios and which in the cold rolled-continuously annealed and hot dip metallic coated condition exhibits high tensile elongation values and a high average plastic strain ratio.

It is a further object to produce a steel having the above properties with substantial reduction in columbium and titanium, or columbium and zirconium, additions as compared to the amount of each element required to achieve comparable properties if used alone.

The objectives of the invention are achieved by the provision of a vacuum degassed and deoxidized low carbon steel to which columbium, and titanium or zirconium additions are made in conformity with the following relationships:

When titanium and columbium are used, the amount of titanium must be equal to or lessthan 4- X wt. percent carbon 3.43 X wt. percent nitrogen, except titanium as titanium oxides. Thisqnay be expressed as:

[Ti/C+ (12/14) N] 4/1 except Ti as Ti oxides [Ti/C (12/14) N] 4/1 or the amount of columbium must be:

greater than 0.0025 wt. percent 7.75 [wt. percent total (wt. percent Ti'- 3.43 wt. percent N)/(4)] if [Ti/C+ (12/14) N] 4/1 provided lwt. percent C (wt, percent Ti 3.43 wt. percent 0.003 m 0.004 wt. percent 6.

1n (2b) above it will be recognized that the factor following 0.025 wt. percent represents the amount of columbium required to combine with that portion of the total carbon not already combined with titanium. As will be shown hereinafter, the precipitation hardening effect of columbium carbides is avoided if less than 0.003 to 0.004 wt. percent of carbon is so combined.

When zirconium and columbium are used, the amount of zirconium must be equal to or less than 7.6 X wt. percent carbon 6.51 X wt. percent nitrogen, except zirconium as zirconium oxides and zirconium sultides. This may be expressed as:

[Zr/C+ (12/14) N] 7.6/1

except Zr as Zr oxides and Zr sulfides, where 12 is the atomic weight of C and 14 is the atomic weight of N. The amount of columbium must be:

greater than 0.025 wt. percent if 4 [Zr/C+ (12/14) N] 7.6/1

or the amount of columbium must be greater than (4) 0.025 wt. percent 7.75 [wt. percent C (wt. percent Zr 6.51 wt. percent N)/(7.6)]

if [Zr/C+ (12/14) N] 7.6/1 provided [wt. percent C (wt. percent Zr 6.51 wt. percent )l 0.003 to 0.004 wt. percent C. 1n (4b) above it will be recognized that the factor following 0.025 wt. percent represents the amount of columbium required to combine with that portion of the total carbon not alreadycombined with zirconium. The wt. percent of Zr excludes Zr oxides and Zr sulfides.

If the compositions of the alloys conform to the requirements set forth above in (1), (2a) or (2b), or in (3), (4a) or (4b), the steels will have the following characteristics: 7

As hot rolled, the precipitation hardening effects observed in columbium-treated steels are eliminated. In this connection it should be noted that precipitation hardening is associated with columbium carbide formation in columbium treated steels. It has now been found that the addition of titanium or zirconium in combination with columbium results in the preferential formation of titanium or zirconium carbides rather than columbium carbides. The hot rolled thin bar will thus have properties substantially independent of the coiling temperature utilized in hot rolling and substantially independent of total carbon content. The properties of principal interest are:

absence of yield point elongation, the presence of which produces undesirable coil breaks and fluting;

good formability anddrawability associated with low yield strengths, high tensile to yield strength ratios, and good ductility. While steels treated only with titanium will have comparable properties in the hot rolled condition, the steels of this invention require substantially less titanium than those containing titanium alone. Since the titanium recovery is relatively low (generally -70 percent), it will be apparent that less total loss occurs at the lower levels of addition required in the present invention, thereby resulting in lower cost.

As cold rolled and batch annealed, the addition of titanium or zirconium in combination with columbium results in tensile elongation values superior to, and average plastic strain ratio values equivalent to, steels containing columbium alone. The properties are characterized by:

absence of yield point elongation in the annealed condition;

high r,,, values, resulting in extra-deep drawing qualhigh tensile to yield strength ratios;

excellent tensile elongation values.

The steels of the invention exhibit a finer grain size than steels treated with titanium alone. This is advantageous for some applications of cold rolled and batch annealed material, e.g., avoidance of orange peel surface on drawn parts where appearance is important such as chromium plated parts requiring jewelryquality finish.

As cold rolled and continuously annealed, or continuously annealed and hot dip metallic coated, the addition of titanium or zirconium in combination with columbium results in tensile elongation values markedly superior to, and average plastic strain ratio values superior to, steels containing columbium alone. The properties are characterized by:

absence of yield point elongation in the annealed condition;

high r,,, values, resulting in extra-deep high tensile to yield strength ratios;

excellent tensile elongation values.

In its broadest range the steel of the invention has the following composition in the ingot or hot rolled band stage, all the percentages being by weight:

drawing qual- Columbium 0.025 to 0.12%

Titanium about 0.015 to 0.12%, except Ti as Ti oxides Zirconium about 0.028 to 0.18%, except Zr as Zr oxides & sulfides Carbon about 0.002 to 0.020%

Nitrogen up to about 0.008%

Manganese up to about 0.60%

Sulfur up to about 0.035%

Oxygen (total) up to about 0.010%

Aluminum (total) up to about 0.045% Phosphorus residual Silicon residual Balance substantially iron, except for incidental impurities In the above steel, all the nitrogen is combined as titanium or zirconium nitrides, and all the carbon exceeding 0.003 to 0.004 percent is combined as titanium or zirconium carbides. When zirconium is used, all the sulfur is combined as a zirconium sulfide.

The composition range in the cold rolled and annealed stage will be essentially the same as stated above for the ingot or hot rolled band stage. However, it should be noted that where the material will be subjected to processing conditions tending to cause pickup of nitrogen (e.g., tight coil annealing of cold rolled strip in a hydrogen-nitrogen atmosphere), it is within the scope of the invention to add sufficient excess titanium or zirconium to the molten charge to scavenge the anticipated nitrogen pick-up and thus prevent any substantial formation of free nitrogen in the final prodnet.

The steel of the invention is produced by melting a charge of steel in any conventional manner having a maximum carbon content of about 0.05 percent, vacuum degassing the steel to a carbon content of about 0.020 percent maximum, an oxygen content of about 0.010 percent maximum, and a nitrogen content of about 0.008 percent maximum, adding titanium or zirconium in an amount calculated to be sufficient to react with all the carbon, nitrogen and oxygen (plus sulfur in the case of zirconium), adding columbium in an amount sufficient to produce greater than 0.025 percent columbium in solid solution in the hot rolled stage, as determined by sheet analysis at room temperature. The degassed steel is then cast into ingots or strand cast, solidified, hot rolled to band thickness finishing at conventional temperatures of about 816 C to about 927 C, and coiled in accordance with conventional practice. The hot rolled product will then ordinarily be pickled and cold reduced to final gauge, and subjected to a final anneal at about 705 to 788 C in a batch anneal, or up to 900 C strip temperature in a continuous anneal.

The degassing step includes deoxidizing by addition of sufficient aluminum to eliminate excessive evolution of gases in advance of the columbium and titanium or zirconium additions. Silicon or titanium could also be substituted for aluminum at this stage as a deoxidant.

It will be apparent that the present invention differs from the above-mentioned Shimizu et al. US. Pat. No. 3,522,110 in requiring titanium (or zirconium) in combination with columbium, with the titanium content being equal to or less than 4 times the carbon content plus 3.43 times the nitrogen content. The patentees disclose a composition containing 0.001 to 0.020 percent carbon and 0.02 to 0.5 percent titanium (except titanium as oxides), with the titanium content being greater than four times the carbon content. In the Shimizu patent there are no steels disclosed as being within the invention in which the titanium content is equal to or less than four times the carbon content plus 3.43 times the nitrogen content. In the abovementioned Forand U.S. Pat. No. 3,607,456, a minimum of 0.15 percent titanium is required in a steel having a maximum carbon of 0.020 percent and a maximum nitrogen of 0.010 percent, with the titanium content being a minimum of seven times the carbon plus nitrogen contents. In contrast to this, the present invention requires titanium (or zirconium) and columbium, with a maximum titanium content of 0.12 percent and equal to or less than four times the carbon content plus 3.43 times the nitrogen content.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the composition has been set forth in broad ranges, preferred and more preferred ranges, resulting in an optimum combination of properties, are as follows, all percentages being by weight: I

Preferred 0.025 to 0.060% 0.015 to 0.061%

More preferred 0.025 to 0.040% 0.015 to 0.045%

Columbium Titanium (except Ti as Ti oxides) or Zirconium (except Zr as Zr sulfides and 0.028 to 0.12% 0.028 to 0.085%

incidental impurities Preferably, in the above alloys, when titanium is used:

[Ti/C (12/14) N] 4/1 0.05 0.0241 0.0270 0.0311 0.000 0.06 0.0189 0.0224 0.0259 0.006 except Ti as Ti ox1des; 0.03 0.0465 0.0500 0.0535 0.009 0.04 0.0413 0.0448 0.0483 0.009 and when mcomum 0.05 0.0361 0.0396 0.0431 0.009 0.06 0.0309 0.0344 0.0379 0.009 [Zr/C+(12/l4)N]=7.6/l 5 except Zr as Zr oxides and sulfides. The relationship of composition, and particularly the If zirconium is added in excess of 7.6 X wt. percent amount of uncombined columbium, to the properties carbon 6.51 X wt. percent nitrogen, it will combine ofa titanium and columbium-treated steel with varying with sulfur in the weight ration of L42 zirconiumzl sulcarbon contents was investigated. The carbon content fur whether or not sufficient manganese is present to in an ingot of mill-produced steel was increased from combine with sulfur. Since the present invention conbutt to top of the ingot by employing a carbon-bearing templates subjecting the molten charge to vacuum dehot topping compound after teeming. The analysis and oxidation and substantially complete deoxidation with properties at varying carbon levels are set forth in aluminum or titanium, the amount of zirconium oxides Table l below.

TABLE I Lab. cold rolled 60% lab. anneal,

Mill hot 732 C.-4 hr.

rolled,

percent Percent Ti as Ti as Ti 1111- Ch as Cb un- C Cb Ti C N S Mn Al YPE YPE r-90 'IiN TiC comb. CbC comb. COIlIllD 028 043 "2 0130 0036 013 Z8 045 5 l2 1. 18 0124 0300 0 028 0 0017 i. 0084 0 1. a0 .0121 .0300 0 .0002 0213 0 n 0076 0 1. 7a .0124 .0300 0 0 .028 0 0072 0 1. 77 012-1 .020; .0010 0 02s 0 Ladle analyses.

formed will be negligible. 7

Although titanium and zirconium have substantially equivalent functions when added with columbium, from what has been said above it will be apparent that there are some differences. It has been discovered that, unlike columbium, titanium and zirconium do not produce a precipitation hardening effect. On the other hand, titanium has only a very slight effect in retarding recrystallization, whereas zirconium has a strong effect in retarding recrystallization, comparable to that of columbium.

Zirconium scavenges carbon, nitrogen and sulfur in the presence of columbium, manganese and aluminum. Titanium behaves similarly with respect to carbon and nitrogen. Titanium is a stronger carbide former than columbium. However, both titanium and zirconium preferentially react with nitrogen before carbon.

Within the above preferred composition ranges, exemplary compositions may be calculated in accordance with formulas (l) and (2a) or (2b), or (3) and (4a) or (4b), which will exhibit the desired properties. By way of illustration a tabulation is set forth below for titanium and columbium additions wherein the total carbon, nitrogen and columbium weight percents are given; the weight percent titanium includes the amount available to form carbides and nitrides, but excludes titanium as titanium oxides.

For 'For For Cb 0.003% N 0.004% N 0.005%N For Required of Ti 'l'i Ti C required required required 0.03 0.0225 0.0260 0.0295 0.003 0.04 0.0l73 0.0208 0.0243 0.003 0.05 0.0122 0.0157 0.0192 0.003 0.06 0.0070 0.0l05 0.0140 0.003 0.03 0.0345 0.0380 0.0415 0.006 0.04 0.0293 0.0328 0.0363 0.006

It will be' noted that the highest carbon sample, wherein there was no uncombined titanium and columbium but 0.0017 percent uncombined carbon, exhibited a substantial yield point elongation both in the hot rolled and cold rolled and annealed conditions. In contrast to this, the sample containing 0.0076 percent carbon, with 0.028 percent columbium in solid solution, exhibited no yield point elongation either in the hot rolled or cold rolled and annealed conditions, and exhibited a marked increase in the transverse r value. In this connection it may be explained that the absolute r,, value for this sample would be about 2 if it had been subjected to mill cold rolling and annealing. The magnitude of the individual r values are not significant, but the differentials between the first and last two samples demonstrate the elimination of yield point elongation and the marked increase in r values resulting from the presence of more than 0.025 percent columbium in solid solution.

In Table l the partitioning of titanium as T iN and TiC and columbium as CbC was derived as follows:

Ti/N in 101v 47.90/14 I 3.43 Ti/C in TiC 47.90/12= 4.0 Cb/C in CbC 92.91/12 7.75

Table ll Continued Percent Coiling 0.5% elongatemp., Y.S., T.S., tion in Cb Ti N C. MN/m 1 MN/m 1 5 cm.

In all the above heats the yield point elongation of the hot rolled thin bar was 0 percent. The oxygen content of all heats was typical of vacuum degassed material and averaged about 0.003 percent.

The columbium and titanium-treated steels and the columbium-treated steels in Table 11 are listed in the order of increasing carbon contents, respectively. It will be noted that carbon contents ranging from 0.0022 to 0.018 percent and coiling temperatures ranging from 649 to 726 C had very little effect on the tensile and elongation properties of hot rolled columbium and titanium-treated steels. In contrast to this, in a columbium-treated steel having a carbon content above about 0.005 percent low coiling temperature causes precipitation hardening. However, at lower carbon'levels coiling temperature has little effect on the hot rolled properties of columbium-treated steel.

Partial analyses and properties of cold rolled and batch annealed columbium and titanium-treated steels of the present invention are set forth in Table III below. Nominal percent cold reduction was carried out in all samples.

Partial analyses and properties of cold rolled, continuously annealed'and hot dip galvanized columbium and titanium-treated steels of the invention are given in Table IV below. For purposes of comparison several cold rolled, continuously-annealed and hot dip galvafilled 'cqlvm mea ,st are a so included- TABLE III 0.5% Percent 7 Sample location in Y.S. T.S. elong. Heat Cb Ti C N 0011 MN/m. 2 MN/m. 2 1n 5 cm.- I rm" 800555 ('Ii deoxidized, 0% temper) .12 .062 .0035 .0053 'I (long. and trans.) 163 308 48.0 1.94 .12 .064 0038 .0038 T (long. and trans)... 156 320 43.1 1.84 800556, 0% temper .066 .078 .00 .0050 F 131 305 48. 8 2.03 .060. .0025 M. 298 48. 3 1. 04 067 075 0020 131 294 50. 0 2. 09

210644 (strand east) (ladle analysis) 0% temper .064 .051 .000 .0052 4T (long) 181 300 43. 8 1.96 .064 .051 .009 .0052 4F Gong.) 128 I 312 46.5 1.90 1254431, 0.5% temper .051 .081 .0055 .0031 T 168 311 48.0 1.05 125-1431, 0.7% temper .051 .081 .0058 0031 T 160 303 49.0 2.03 205 308 48.0 1. 94

151 300 46. 0 1. 92 125 201 49.0 2.07 134 301 48.0 2. 05 T=Tail of strip (ingot butt); F=Front of strip (ingot top); M=.\li(ldle of strip. "rm=% Ir(longitudinal r(tronsverse) 2r(diagonal)]. Nor1-:. Yield point elongation=0% in all samples.

TABLE IV Sample 0.5% Percent location Y.S., T.S., elong. Heat Cb T: C N in coll MN/m. MN/m.2 in 5 cm. r 800555 (Ti deoxidized), 0.7% temper 0.12 0.062 0. 0035 0. 0035 IT 105 222 43.5 1. 94 4T 172 309- 44.0 2.17 800556, 0.7% temper 0. 060 2T 138 295 46.5 2. 42 169 295. 46. 0 2. 11

2260113, 0% temper 124 285 49. 5 2.16. 116 290 47. 0 2. 18

1251270, 0% temper 123 293 45. o 2.06 129 296 47. 5 2. 10

2250618 (ladle analysis) 0% temper 0.028 0.038 0 004 0 0042 F 131 292 43 1.92 490376, 0% temper... O 183 328 39. 5 1.80 400854, 0% temper 158 328 38. 5 1. 75 336 40. 0 1. 77 100254, 1% temper 216 320 42. 0 1. s0 240 333 41. 5 1. 76

400853 ,1% temper 0 11 210 326 40.5 1.79 224 327 40. 5 1. 73 230 332 41.0 1. 69

It will be apparent from Table 1V that the columbium and titanium-treated steels of the invention exhibit tensile elongation and r,, values superior to those of columbium-treated steels.

The composition and properties of a columbium and zirconium-treated heat are set forth below in Tables V and V1, respectively.

In those cold rolled and annealed samples where grain size was measured, it was found to range between ASTM grain sizes 8 and 10.

An investigation of the recrystallization response of the columbium and titanium-treated steels of the invention in comparison to steels containing titanium only showed that the presence of columbium in solid solu- TABLE V A1 Product and sample location Cl) Zr C N O S Mn (total) 0. 066 0. 044 0. 0077 0. 0072 O. 0015 0. 021 0, 3 0. 06 0. 067 0. 048 0. 0053 0. 0060 0. 0062 O. 021 O. 3 0. 06

TABLE VI Percent 0.5% Percent yield Sample Y.S., T.S., elong. pain 1: Product location MN/rn. MN/m. in 5 cm. 1... elong.

F 292 400 35. 8 1. Hot rolled "11* 206 354 41. 0

. 9 Continuously annealed and galv niz temper 382 Z2? i'g 8 Cold rolled and batch annealed, 0.3% temper It will be noted that the front sample of the hot rolled thin bar exhibited 1.0 percent yield point elongation.

HOT ROLLED Tail .039 Zr as ZrN (.0060N) .009 Zr as ZrC (.0012C) .048 Zr total Front .044 Zr as ZrN (.0067N) .044 Zr total balance of N(.0005N) as AlN .0045 o as A003 .0062 o as 70,0,

.021 S as MnS .021 S as MnS .0596 Cb as CbC(.0077C) .032 Ch as CbC(.004lC) .066 Cb total .067 Cb total .0064 Cb uncombined %YPE 1.0%

60% lab. cold rolled and lab annealed r,, 1.57

.035 Cb uncornbined %YPE 0 60% lab. cold rolled and lab. annealed r, 1.67

CONTINUOUSLY ANNEALED & GALVANIZED Tail .0397 Zr as ZrN (.0061N) .0103 Zr as ZrC (.0013C) Front .044 Zr as ZrN(.0067N) .006 Zr as ZrC(.0008C) .05 Z.r total .05 Zr total .0074 O as A1 0, .010 O as ALO .021 Sas MnS .019SasMnS .039 Cb as CbC (.OOSOC) .020 Cb (.0026C) .066 Cb total .064 Cb total .027 Cb uncombined .044 Cb uncombined kYPE 0 %YPE 0 From the above calculations it is apparent that when the amount of uncombined columbium is less than 0.025-wt. percent (hot rolled-front sample) the columbium and zirconium-treated steel has a yield point elongation and relatively low r, value. 1n all other samples, wherein the uncombined columbium ranges from 0.027 to 0.044 percent, the product has no yield point elongation and thus is non-aging. Y

tion significantly elevates the recrystallization temperature as compared to titanium in solid solution. in Table V11 below two heats containing titanium only and two heats containing titanium and columbium are compared, and it will be noted that an increase in the titanium content with no columbium present had no effect on the recrystallization temperature, whereas progressively increasing columbium contents increased the recrystallization temperature. In all cases recrystallization occurred by the formation of recrystallized grains distributed randomly throughout the cold worked matrix. There was no evidence of progressive recrystallization inwardly from'the sheet surfaces, as occurs typically in columbium-treated steel.

In all heats of Table V11 the R hardness values were less than 40 after complete recrystallization. Hence there is an absence of a precipitation-hardening effect in the steels of the invention.

Modifications may be made without departing from the spirit of the invention. For example, while the specific examples disclose columbium and titaniumtreated steels, or columbium and zirconium-treated steels, it will be evident that mixtures of titanium and zirconium can be added along with columbium. 1n this event, the calculation of the respective proportions of titanium and zirconium is somewhat more complex.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. Non-aging, low carbon steel having substantially no yield point elongation in the hot rolled and cold rolled and annealed conditions, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to about 0.035 percent sulfur, up to about 0.045 percent total aluminum, and balance substantially iron ex-' cept for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than 0.025 percent columbium in uncombined form, and not more than 0.004 percent carbon combined with columbium.

2. The steel claimed in claim 1, consisting essentially of from about 0.002 percent to about 0.010 percent carbon up to about 0.35 percent manganese, from greater than 0.025 percent to about 0.060 percent columbium, at least one of titanium and zirconium, titanium being from about 0.015 percent to about 0.061 percent, zirconium being from about 0.028 percent to about 0.12 percent, from about 0.002 percent to about 0.006 percent nitrogen, up to about 0.004 percent total oxygen, up toabout 0.020 percent sulfur, up to about 0.010 percent phosphorus, from about 0.015 percent to about 0.020 percent total aluminum, up to about 0.015 percent silicon, and balance substantially iron except for incidental impurities.

3. The steel claimed in claim 1, consisting essentially of from about 0.002 percent to about 0.006 percent carbon, up to about 0.35 percent manganese, from greater than 0.025 percent to about 0.040 percent columbium, at least one of titanium and zirconium, titanium being from about 0.015 percent to about 0.045 percent, zirconium being from about 0.028 to 0.085 percent, from about 0.002 percent to about 0.006 percent nitrogen, up to about 0.004 percent total oxygen, up to about 0.01 percent sulfur, up to about 0.010 percent phosphorus, from about 0.015 percent to about 0.020 percent total aluminum, up to about 0.015 percent silicon, and balance substantially iron except for incidental impurities.

4. Hot rolled thin bar having no yield point elongation, good formability and drawability and high tensile to yield strength ratios, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up

to about 0.035 percent sulfur, up to about 0.045 percent total aluminum, and balance substantially iron except for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than'0.025 percent columbium in uncombined form, and not more than 0.004 percent carbon combined with columbium.

5. Continuously annealed and hot-dipped metallic coated steel strip having no yield point elongation, good tensile elongation, and high plastic strain ratio, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to about 0.035 percent sulfur, up'to about 0.045 percent total aluminum, and balance substantially iron, except for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than 0.025 percent columbium in uncombined form, and not morethan 0.004 percent carbon combined with columbium. h i ,7 W

6. A cold reduced and annealed steel product having no yield point elongation, high plastic strain ratio, high tensile to yield strength ratio, and good tensile elongation, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to about 0.035 percent sulfur, up to about 0.045 percent total aluminum, and balance substantially iron except for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than 0.025 percent columbium in uncombined form, and not more than 0.004 percent carbon combined with columbium.

7. A method of producing a non-aging, low carbon steel having substantially no yield point elongation in the hot rolled and cold rolled and annealed conditions, which comprises melting a charge of steel having a maximum carbon content of about 0.05 percent, vacuum degassing the steel to obtain a melt having a carbon content of about 0.020 percent maximum, a total oxygen content of about 0.010 percent maximum, a nitrogen content of about 0.008 percent maximum, up to about 0.6 percent manganese, up to about 0.035 percent sulfur, and balance substantially iron, deoxidizing by addition of a deoxidant chosen from the class consisting of aluminum, titanium and silicon, adding at least one of titanium and zirconium, titanium when used being added in an amount sufficient to obtain a titanium content in the hot rolled stage within the range of about 0.015 percent to about 0.12 percent, and zirconium when used being added in an amount sufficient to obtain a zirconium content in the hot rolled stage within the range of about 0.028 percent to about 0.18 percent, adding columbium in an amount sufficient to produce greater than 0.025 percent columbium in solid solution in the hot rolled stage as determined by sheet analysis at room temperature, casting and solidifying said steel, hot rolling to band thickness with a finishing temperature of at least about 816 C, and coiling, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times '16 the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium aszirconium sulfide and zirconium oxide, and not more than 0.004 percent carbon being combined with columbium.

8. The method of claim 7, including the steps of pickling the hot rolled band, cold rolling to final gauge, and batch annealing at a temperature of about 705 to about 788 C.

9. The method of claim 8, including the steps of cleaning the surfaces of the cold rolled material, and applying a-metallic coating.

10. The method of claim 8, including the steps of cleaning the surfaces of the cold rolled material, and applying a non-metallic coating.

11. The method of claim 7, including the steps of pickling the hot rolled band, cold rolling to final gauge, cleaning the surfaces of the cold rolled material, and continuously annealing.

12. The method of claim 11, including the step of applying a metallic coating.

13. The method of claim 11, including the step of applying a non-metallic coating.

Patent No.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated October 16, 1973 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 40, [Ti/C l2/14)N] should be s mi c l2/l4N]- Column Column 3,

Column Column Column Column Column Coluxm 7 Column 7,

[SEAL] line line

line

line

line

line

lin

line

Arrest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN '(ummissioncr oj'Parenls and Trademarks 

2. The steel claimed in claim 1, consisting essentially of from about 0.002 percent to about 0.010 percent carbon up to about 0.35 percent manganese, from greater than 0.025 percent to about 0.060 percent columbium, at least one of titanium and zirconium, titanium being from about 0.015 percent to about 0.061 percent, zirconium being from about 0.028 percent to about 0.12 percent, from about 0.002 percent to about 0.006 percent nitrogen, up to about 0.004 percent total oxygen, up to about 0.020 percent sulfur, up to about 0.010 percent phosphorus, from about 0.015 percent to about 0.020 percent total aluminum, up to about 0.015 percent silicon, and balance substantially iron except for incidental impurities.
 3. The steel claimed in claim 1, consisting essentially of from about 0.002 percent to about 0.006 percent carbon, up to about 0.35 percent manganese, from greater than 0.025 percent to about 0.040 percent columbium, at least one of titanium and zirconium, titanium being from about 0.015 percent to about 0.045 percent, zirconium being from about 0.028 to 0.085 percent, from about 0.002 percent to about 0.006 percent nitrogen, up to about 0.004 percent total oxygen, up to about 0.01 percent sulfur, up to about 0.010 percent phosphorus, from about 0.015 percent to about 0.020 percent total aluminum, up to about 0.015 percent silicon, and balance substantially iron except for incidental impurities.
 4. Hot rolled thin bar having no yield point elongation, good formability and drawability and high tensile to yield strength ratios, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to about 0.035 percent sulfur, up to about 0.045 percent total aluminum, and balance substantially iron except for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than 0.025 percent columbium in uncombined form, and not more than 0.004 percent carbon combined with columbium.
 5. Continuously annealed and hot-dipped metallic coated steel strip having no yield point elongation, good tensile elongation, and high plastic strain ratio, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to about 0.035 percent sulfur, up to about 0.045 percent total aluminum, and balance substantially iron, except for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than 0.025 percent columbium in uncombined form, and not more than 0.004 percent carbon combined with columbium.
 6. A cold reduced and annealed steel product having no yield point elongation, high plastic strain ratio, high tensile to yield strength ratio, and good tensile elongation, consisting essentially of from about 0.002 percent to about 0.020 percent carbon, up to about 0.60 percent manganese, from greater than 0.025 percent to about 0.12 percent columbium, at least one of titanium and zirconium, titanium when present being from about 0.015 percent to about 0.12 percent, zirconium when present being from about 0.028 percent to about 0.18 percent, up to about 0.008 percent nitrogen, up to 0.010 percent total oxygen, up to about 0.035 percent sulfur, up to about 0.045 percent total aluminum, and balance substantially iron except for incidental impurities, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, there being greater than 0.025 percent columbium in uncombined form, and not more than 0.004 percent carbon combined with columbium.
 7. A method of producing a non-aging, low carbon steel having substantially no yield point elongation in the hot rolled and cold rolled and annealed conditions, which comprises melting a charge of steel having a maximum carbon content of about 0.05 percent, vacuum degassing the steel to obtain a melt having a carbon content of about 0.020 percent maximum, a total oxygen content of about 0.010 percent maximum, a nitrogen content of about 0.008 percent maximum, up to about 0.6 percent manganese, up to about 0.035 percent sulfur, and balance substantially iron, deoxidizing by addition of a deoxidant chosen from the class consisting of aluminum, titanium and silicon, adding at least one of titanium and zirconium, titanium when used being added in an amount sufficient to obtain a titanium content in the hot rolled stage within the range of about 0.015 percent to about 0.12 percent, and zirconium when used being added in an amount sufficient to obtain a zirconium content in the hot rolled stage within the range of about 0.028 percent to about 0.18 percent, adding columbium in an amount sufficient to produce greater than 0.025 percent columbium in solid solution in the hot rolled stage as determined by sheet analysis at room temperature, casting and solidifying said steel, hot rolling to band thickness with a finishing temperature of at least about 816* C, and coiling, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, and not more than 0.004 percent carbon being combined with columbium.
 8. The method of claim 7, including the steps of pickling the hot rolled band, cold rolling to final gauge, and batch annealing at a temperature of about 705* to about 788* C.
 9. The method of claim 8, including the steps of cleaning the surfaces of the cold rolled material, and applying a metallic coating.
 10. The method of claim 8, including the steps of cleaning the surfaces of the cold rolled material, and applying a non-metallic coating.
 11. The method of claim 7, including the steps of pickling the hot rolled band, cold rolling to final gauge, cleaning the surfaces of the cold rolled material, and continuously annealing.
 12. The method of claim 11, including the step of applying a metallic coating.
 13. The method of claim 11, including the step of applying a non-metallic coating. 